The present invention relates generally to transmitters and transceivers for high speed communication systems and, more particularly, to transmitters for transmitting signals with a controlled impedance load.
Also, the present invention relates to termination circuits and methods therefor. More particularly, the present invention relates to termination circuits that advantageously provides fast and efficient signal transmission via transmission lines in electronic systems.
In the design and implementation of electronic systems particularly those employing integrated circuits, undesired transmission line effects are of a particular concern. As signals travels down transmission lines, e.g., traces on a printed circuit board, reflections may occur on the lines. The reflections are due to, for example, mismatched impedances between the driver circuit and the line, which may cause the signal to reflect back and forth, giving rise to ringing. These reflections and other undesired transmission lines effects are often exacerbated as the operating speed of the signal increases. If left uncorrected, the reflections may cause the signal""s voltage to swing outside of the defined xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d voltage levels, thereby causing the receiving device to incorrectly interpret the signal received and generate erroneous results.
To address the impedance mismatch between the driver (or receiver) circuit and the transmission line, a variety of techniques have been known in the prior art.
Typically, high speed communication systems transmit a signal down a line with a terminating voltage and terminating resistance, normally located near the receiver. The effect of the transmitter is to send a fully saturated digital signal into the transmission line at the same impedance as the line itself such that any reflections from impedance anomalies in the line do not cause further reflections from the transmitter. The receiver has a terminating load at the same impedance as the transmission line. The terminating resistances are arranged in very high speed systems to create a terminating voltage that is typically the median of the voltage value of the 1 and 0 of the digital signal.
As shown in FIG. 1, to provide the same preset voltage potential to the load, the conventional transceiver may comprise a resistor 3 incorporated in the bus line after the driver 1 and a terminating resistor 4 incorporated into the receiving circuit.
While this approach is effective for some systems, there are numerous disadvantages. For example, the use of an impedance matching resistor attenuates the signal, which lowers noise immunity while dramatically increasing power dissipation. To illustrate, the presence of resistor 4 forms essentially a resistor divider circuit between the characteristic impedance of the transmission line and resistor 4, thereby lowering the signal""s voltage level at the receiving circuit.
Further, the presence of resistor 4 increases the power dissipation, which increases the load on the driver circuit and causes additional heat to be generated as well as more power from the system""s supply.
The presence of resistor 3 creates a situation in which the input to any other device which might be connected to the transmission line sits at half amplitude, an undesirable condition while the line is settling. Still further, it is often difficult to provide impedance matching for transmission lines whose characteristic impedance may change with the system""s configuration.
Thus, for high speed communication systems, it is complex to provide termination voltages and resistors to transmitters, as a receiver requires the signal with a controlled impedance load linked to a terminating voltage, but the transmitter requires to drive with a series termination into the same line. This causes the increase in swing and power losses and the reduction of the communication speed.
Moreover, contemporary high speed communication systems require quick direct triggering. However, the time required to switch the driver circuit of FIG. 1 into tri-state will be bigger than the time of switching from one logical state to another.
In U.S. Pat. No. 5,781,028, a system and a method are described for a switched termination of a universal serial data bus between first and second devices having transmitting and receiving modes. When the first device is in transmitting mode, a second termination coupled within a receiver circuit at the second end of the bus is activated to terminate the second end; when the first device is in receiving mode, a first termination coupled at this end of the bus is activated to terminate the first end. This arrangement provides for activating the termination at the receiving end only.
However, similar to the prior art already described above, switches used for connecting and disconnecting the resistors will introduce additional errors in the system, as the time required to switch the resistors will be bigger than the time of switching from one logical state to another. This prevents from achieving high speed operation in various applications where timings are critical. In view of the foregoing, there are desired transmitters with improved termination circuits and methods therefor that advantageously provide fast and efficient transmission of signals via transmission lines in electronic systems, particularly signals having low operating voltage ranges, whilst at the same time simplifying the implementation of transceivers in very high speed systems.
The present invention provides a transmitter which avoids or at least alleviates the above mentioned disadvantages of the prior art.
According to one aspect of the present invention, a transmitter comprises a plurality, preferably two, drivers each having series terminating resistor. The transmitter further comprises a control unit provided by a data signal and a control signal.
The control unit operates in two modes. In the first mode, the control unit supplies drivers with a signal which is a function of input data, to provide the transmitter drives a communication line with an output impedance equal to the parallel effect of all the terminating resistors.
In the second mode, the control unit generates a signal to make the drivers with respective series terminating resistors acting as a parallel termination circuit. No actual data is transmitted in this mode. In a preferable embodiment, to achieve this, one driver or a group of drivers drives one logic level signal, while the other drivers drives the inverse to this signal.
This means that if all drivers drive a data signal, which, in general, may not be the same for all drivers, then the signal is transmitted with an impedance equal to the parallel effect of all the terminating resistors. For example, if there are two drivers and each resistor is 100 Ohms, then driving the two in parallel will create a 50 Ohm drive impedance into the transmission line.
If the drivers drive complementary signals, that is, for example, one driver is high and the other is low, then a potential divider is formed which has an impedance of 50 Ohms and acts as a terminating resistance and terminating voltage to the receiver.
In another aspect, a method of high speed data transmission is provided, the method comprising the steps of:
(a) providing a plurality of drivers each having series terminating resistor, wherein the resistors are connected at a common point;
(b) providing a data signal;
(c) providing a control signal defining two modes of operation:
(d) in a first mode, a control unit supplies drivers with a signal which is a function of input data, to provide the plurality of drivers drives a communication line with an output impedance equal to the parallel effect of all the terminating resistors;
(e) in a second mode, the drivers are provided with a signal that makes the drivers with respective series terminating resistors acting as a parallel termination circuit, while no actual data is transmitted in this mode.
Preferably, the series terminating resistors are joined at the transmission line.
In case of two drivers, in the first mode they both drive the same signal. In the second mode, the drivers drive different, e.g. complementary, signals, i.e. one driver drives one logic level signal, while the other driver drives the inverse to this signal, thereby, a terminating voltage is formed as a half sum of high and low signal levels (for two equal series resistors).
The combination of the above features enables true synchronous operation of bi-directional communication channels, such as in chip-to-chip communications.